We have shown that 3-day lindane exposure at doses of 20 and 40 mg/Kg/day suppressed bone marrow cellularity, erythrocyte precursors (CFU-E), granulocyte-macrophage progenitor cells (CFU-GM), and residual progenitor cell damage could be demonstrated by whole body irradiation (two WBI 200 rads). Male B6C3F1 mice given lindane daily at doses of 0, 10, or 20 mg/Kg body weight by gavage in corn oil for 10 consecutive days did not show clinical abnormality of changes in body weights, but there was a dose-dependent decrease in marrow cellularity, in more pluripotent stem cells (CFU-S), and in committed CFU-GMs which returned to control values by four weeks. The mice were then subjected to two 100-rad exposures of WBI at four and nine weeks following cessation of lindane treatment. This level of irradiation caused only a transient drop in marrow progenitor numbers. Control and lindane-exposed mice were examined at one and six weeks following the last irradiation, which was 10-15 weeks following the final lindane exposure. The lindane- exposed mice had lower progenitor cell numbers and slower recovery from the irradiation. These results indicate that lindane has significant myelotoxicity in mice and short-term lindane exposure can induce residual progenitor cell damage that can be demonstrated by subsequent irradiation.